Method and apparatus for the transmission of packets of data

ABSTRACT

A method of transmitting packets of data in a communication network comprising at least first to third stations, the method comprising the steps of sending a first number of the data packets from the first station to a second station; identifying which of the first number of packets are correctly received by the second station from the first station; and sending a second number of data packets from the third station to said second station commencing with the packet identified as being required after the last correctly received packet received form the first station.

This is a Continuation of application Ser. No. 09/765,144 now U.S. Pat.No. 7,012,905, filed Jan. 18, 2001, which in turn is a Continuation ofInternational Application No. PCT/EP99/04979, filed Jul. 14, 1999, bothof which are incorporated by reference in their entirety herein.

The present invention relates to a method and apparatus for transmittingpackets of data in a wireless communication network and, in particular,but not exclusively in a cellular telecommunication network.

In wireless communication systems such as cellular telecommunicationnetworks, the area covered by the network is divided into a number ofcells 2, such as shown in FIG. 5. Each cell 2 is served by a basetransceiver station BTS which transmits signals and receives signalsfrom terminals MS located in the associated cell. These terminals MS maybe mobile stations and able to move between the cells 2. The mobileterminals MS will thus communicate with the base transceiver station BTSserving the cell in which the particular mobile station is located. Whena mobile station MS moves from one cell 2 to another, handoff (alsoknown as “handover”) will occur. In other words, the mobile station MSwill stop communicating with the base transceiver station BTS in the oldcell and start communicating with the base transceiver station BTS inthe new cell.

For the communication of data between mobile stations MS and basetransceiver stations BTS, packet data transmission techniques aresometimes used. The data to be transmitted is divided into packets whichalso include information such as the address of the destination to whichthe packet is being sent and the identity of the packet such as itsnumber. In known systems, it has been a problem that handoff can causethe loss of packets of data. For example, the first base transceiverstation BTS will receive packets of data to be sent to a given mobilestation MS. When it is determined that handoff is to take place, thepackets of data will then be sent to the new base transceiver stationBTS. The problem arises in that handoff may be completed before thefirst base transceiver station BTS has finished transmitting all of thedata packers which it has received to the mobile station MS.

These untransmitted packets will then be lost.

It has therefore been proposed to send the same data packets to morethan one base transceiver station. Thus, the same data packets would besent to the first and second base transceiver stations. In thisproposal, the data packets are sent to the first and second basetransceiver stations. In one base transceiver station, a data packet istransmitted to the mobile station whilst in the other base transceiverstation, that same packet is discarded. However, if there is not exactsynchronisation between the receiving of the data packets by each of thebase transceiver stations and the discarding and transmission of a givendata packet, data packets may still be lost. For example, if the secondbase transceiver station discards the, data packets more quickly thanthe first base transceiver station transmits the data packets, then whenhandoff occurs, the second base transceiver station may try to transmitas its first data packet, the n+2th packet whilst the packet lastreceived by the mobile station is the nth packet. In other words, then+1th packet may be lost. This problem could be overcome by transmittingall data packets to a group of neighbouring base transceiver stations.However this proposal has the disadvantage that a high buffer overheadwould be required to store the transmitted data packets. It is preferredthat the buffer overhead is minimised.

It is also desired that the delay in receiving data packets should beminimised.

It is therefore an aim of embodiments of the present invention to removeor at least mitigate the problems of the known arrangements.

According to a first aspect of the present invention there is provided amethod of transmitting packets of data in a communication networkcomprising at east first to third stations, the method comprising thesteps of sending a first number of the data packets from the firststation to a second station; identifying which of the first number ofpackets are correctly received by the second station from the firststation; and sending a second number of data packets from the thirdstation to said second station commencing with the a packet identifiedas being required after the last correctly received packet received fromthe first station.

By identifying which data packets have been received by the secondstation and ensuring that the next data packet which is transmitted bythe third station is the next required packet, it can be ensured thateven during handoff, that the second station receives all of the datapackets. The next required packet may be the packet subsequent to theidentified last packet or may be a data packet which is beingretransmitted, the first transmission of that packet being unsuccessful.

Preferably, at least some of the packets of data to be transmitted tothe second station are provided both to the first and third stations.This has the advantage that when the first station stops transmittingpackets and the third station starts transmitting packets, the thirdstation will already have the necessary packets including the packetfollowing the identified last packet. The wireless network may comprisea common node connected to the first and third stations, said commonnode being arranged to transmit data packets to the first and thirdstations. The method may include the step of acknowledging by the secondstation the correct receipt of the data packet.

According to a second aspect of the present invention, there is provideda method of transmitting packets of data in a communication networkcomprising at least first to third stations and at least one commonnode, said second station arranged to be in communication with at leastone of said first and third stations and said first and third stationsbeing connected with said common node, the method comprising the stepsof indicating to a common node that the second station is reachable viaboth of said first and third stations; transmitting the same datapackets from the common node to the first and third stations;transmitting the said same data packets from at least one of the firstand third stations to said second station; identifying a received packetat the third station and acknowledging receipt of that packet; andremoving said acknowledged packet from data storage means.

Preferably, in both aspects, the data packets are provided both to thefirst and third stations when the second station has a predefinedparameter with respect to the first and third stations. That predefinedparameter may be defined by the geographic position of the secondstation with respect to the first and third stations. Thus, thecircumstances in which the first and third stations receive the samedata packets can be defined with respect to the location of the secondstation with respect to the first and third stations. In a cellularnetwork, the location may be predefined for each cell. It should benoted that the location is preferably a zone.

Alternatively, and more preferably, the at least one predefinedparameter is defined by at least one parameter of signals received fromat least one of the first and third stations satisfying a predeterminedcriteria. The signals may be the data packets received from the firstand/or third stations or alternatively may be some other signals such ascontrol signals, reference signals etc.

The parameter may be the received power level at the second station fromat least one of the first and third stations. For example, when thepower level of a signal received from the first station falls below apredetermined threshold, the beginning of the location may be defined.Likewise, the end of the location may be defined by the power level of asignal received from the third station exceeds a predetermined level.Alternatively, the parameter may be the ratio of the power levels ofsignals received at the second station from the first and thirdstations.

Yet another alternative is that the parameter may be the signal qualityreceived at the second station from the first and/or third stations.Another alternative is for the parameter to be based on the quantity oftraffic, the quality of service and/or quality of the signals.

Preferably, the parameter is averaged over time before it is determinedif the criteria is satisfied. Thus, anomalous readings which couldadversely effect the operation of the system can be ignored.

Preferably, said predefined location includes a handoff zone in whichsaid first station hands off to said third station in that said secondstation stops receiving data packets from said first station and startsreceiving data packets from said third station. Preferably, the handoffzone is surrounded on either side by a zone in which the second stationreceives data packets from a respective one of said first and thirdstations, and said first and third stations are both provided with thedata packets. This ensures that when handoff occurs, the third stationhas the next data packet to be transmitted to the second station.

When handoff takes place, said second station transmits a signal to thethird station to advise the third station of which packet or packetswere received from the first station and said third station transmitsthe data packet identified as being required after the last packet tosaid second station.

Preferably, the data packets are stored in the first and third stationswhen provided thereto. This is of advantage particularly if the datapacket needs to be retransmitted due to an error. Additionally, itensures that when handoff occurs, the third station will have the nextdata packet to be transmitted. When handoff takes place, the datapackets preceding the next data packet to be transmitted may bediscarded in the third station.

Preferably, said first and third stations are base transceiver stationswhilst the second station is a mobile station. Preferably, the basestations and mobile stations are part of a cellular telecommunicationnetwork.

The common node may not be advised of the occurrence of handoff. Thismay be unnecessary if, for example, the first and third stations areboth provided with the data packets.

It is preferred that the common node has a higher hierarchy than thefirst and third stations in the wireless network. However, it ispossible that one of the first and second stations could also providethe common node function.

Preferably, said data storage means is in said common node and/or atleast one of said first and third stations. The acknowledgement may besent to the common node. The data storage means may comprise buffermeans. The common node may associate a unique number with each packetand the same data packets, each of which is associated with the uniquenumber, are transmitted from said common node to both of the first andthird stations. The unique number may be in or associated with thepacket. The common node may control the removal of said identified datapacket from the or each data storage means.

The first station may be in communication with a first node and thethird station may be in communication with a second node, said first andsecond nodes being in communication with said common node. Theacknowledgement may be forwarded to the common node by one of said firstand second nodes and the common node may advise the other of the firstand second nodes that an acknowledgment has been received.

Alternatively, the first station is connected to a first node and thethird station is connected to a second node and one of the first andsecond nodes is arranged to be the common node and said acknowledgementsare forwarded to the common node. Preferably, the first and second nodesare connected together.

Alternatively, the first and third stations may be connected to thecommon node via a network.

Preferably, the acknowledgment of the receipt of a packet is sent by thesecond station to at least one of said first and third stations.

The data storage means may be provided in the first and third stationsand the common node may be arranged to cause a transmitted packet to beremoved from said data storage means of at least one of said first andthird stations on receipt of an acknowledgement that said transmittedpacket has been received by said second station.

The network is preferably a wireless network but may alternatively be awired network.

The communication network may be a general packet radio system (GPRS) inwhich said first and third stations are base stations or base stationcontrollers and said common node is a SGSN. The first and third stationsmay be in communication with different SGSNs, one of said SGSNs beingdesignated as the common node, the other SGSN being arranged to forwardacknowledgements to the common node.

Alternatively, said wireless communication network is a GPRS network andsaid first and second stations are SGSNs and said common node is a GGSN.In a further alternative, the first and third stations are base stationsand said common node is a base station controller.

The communication network preferably has a plurality of cells and/orareas and said second station is able to register with one or more cellsand/or one or more areas of said network at the same time. The decisionas whether or not the second station registers with one or more cells orareas may depend on at least one predefined parameter such as discussedhereinbefore. Data packets may be provided to both of said first andthird stations if the second station is registered with both of thefirst and third stations.

According to a further aspect of the invention, there is provided asystem of transmitting packets of data in a communication networkcomprising:—first, second and third stations, wherein said first stationis arranged to send a first number of the data packets to the secondstation, the second station is arranged to identify the which of thefirst number of packets it receives from the first station, and thethird station is arranged to send a second number of data packets to thesecond station commencing with the data packet identified as beingrequired after the last correctly received packet from the firststation.

For a better understanding of the present invention and as to how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying figures in which:—

FIG. 1 illustrates the principles of embodiments of the presentinvention;

FIG. 2 schematically shows the structure of a data packet;

FIG. 3 shows the power levels of signals received at a mobile stationfrom two adjacent base transceiver stations versus distance;

FIG. 4 shows a schematic view of elements of a cellulartelecommunications network;

FIG. 5 shows a schematic view of a cellular telecommunications network;

FIG. 6 shows an alternative embodiment of the present invention in aGPRS network; and

FIG. 7 shows a modification to the embodiment shown in FIG. 6.

One embodiment of the present invention will be described in the contextof a cellular telecommunications network which uses the GSM (GlobalSystem for Mobile communications) standard.

Reference will first be made to FIG. 1 which shows a first basetransceiver station BTS1 and a second base transceiver station BTS2. Amobile station MS1 is currently in the cell 4 which is associated withthe first base transceiver station BTS1 and is moving towards the cell 6which is associated with the second base transceiver station BTS2. Theapproximate boundary between the two cells 4 and 6 is represented byregion 8. It should be appreciated that, in practice, the two cellsgenerally share the overlapping region 8. That is, the mobile stationMS1, when in this region, is capable of communicating either with thefirst base transceiver station BTS1 or the second base transceiverstation BTS2. This overlapping region a represents the handoff zone. Inother words, at some point in this region, the mobile station MS1 willstop communicating with the first base transceiver station STS1 andstart communicating with the second base transceiver station BTS2.

Embodiments of the present invention are particularly applicable to thetransmission of packets of data from base transceiver stations to agiven mobile station. FIG. 2 illustrates schematically one possiblestructure for a data packet 10 to be transmitted from a base transceiverstation to a given mobile station. The data packet may have a fixedlength or, alternatively may have any suitable length. The data packetincludes address information 12 which defines the mobile station towhich the data packet 10 is to be sent. The data packet 10 also includesinformation 14 relating to the identity of the packet. For example, thisidentifying information 14 may comprise the number of the packet. Itshould be appreciated that the address information 12 and theidentifying information 14 may be of predetermined length and occupypredetermined locations in the data packet. This is followed by the data15 to be transmitted to the mobile station. It should be noted that theorder of the address information 12, the identifying information 14 andthe data 16 may be altered in accordance with the data packet formatbeing used for a particular application.

Reference will now be made to FIG. 4 which schematically shows some ofthe elements of a cellular telecommunications network. As can be seen,the first and second base transceiver stations BTS1 and BTS2 areconnected to a base station controller BSC2. As can be seen from FIG. 4,a third base transceiver station BTS3 is also connected to the same basestation controller BSC2 as the first and second base transceiverstations BTS1 and BTS2. In practice, each base station controller 2 canbe connected to any number of base transceiver stations.

As can be seen, three base station controllers BSC1-3 are shown in FIG.4. It should be appreciated that the first and third base stationcontrollers BSC1 and BSC3 are also each connected, in general, to anumber of base transceiver stations which have not been shown in theinterests of clarity. Each of these base station controllers BSC1-3 isconnected to a mobile switching centre MSC. Again, it should beappreciated that the mobile switching centre MSC can be connected to anynumber of base station controllers. Furthermore, a cellulartelecommunications network will generally include a number of mobileswitching centres MSC. The mobile switching centre receives an input 18which comprises the data packets which are to be transmitted to themobile station MS1. The mobile switching centre MSC passes the datapackets to the base station controller BSC2 which controls the basetransceiver station BTS1 with which the mobile station is currentlycommunicating and possibly the base transceiver station BTS2 with whichthe mobile station MS1 is likely to communicate with in the future. Inthe case of the embodiment shown in FIG. 4, the packets of data will bepassed to the second base station controller BSC2. Depending on thelocation of the mobile station MS1 with respect to the first basetransceiver station BTS1 and the second base transceiver station BTS 2,the base station controller BSC2 will do one of the following:

-   (a) Transmit the data packets only to the first base transceiver    station BTS1;-   (b) Transmit the data packets both to the first and second base    transceiver stations BTS1 and BTS2; or-   (c) Transmit the data packets only to the second base transceiver    station BTS2.

Each base transceiver station BTS1-3 is provided with a memory 20 in theform of a buffer which is able to store the packets of data receivedfrom the base station controller BSC2.

Referring back to FIG. 1 and to FIG. 3, an embodiment of the presentinvention will now be explained. Initially, the mobile station MS1 is inthe cell 4 associated with the first base transceiver station BTS1. Thiscorresponds to the part of the graph indicated by reference number 22 inFIG. 3. As can be seen, the power of the signal received from the firstbase transceiver station BTS1 by the mobile station is very much greaterthan the power level of the signal received from the second basetransceiver station BTS2. Accordingly, the data packets will betransmitted from the base station controller 2 only to the first basetransceiver station 1. The data packets will then be transmitted by thefirst base transceiver station BTS1 to the mobile station MS1.

An automatic retransmission protocol is also used so that if the mobilestation MS1 does not correctly receive a data packet, the mobile stationMS1 advises the first base transceiver station BTS1 of this. The firstbase transceiver station BTS1 will then retransmit the incorrectlyreceived data packet.

The next data packet will only be transmitted once the first basetransceiver station STS1 has received an acknowledgement from the mobilestation MS1 that the previous packet has been correctly received. Ofcourse, if the first base transceiver station STS1 receives anindication from the mobile station MS1 that it has not correctlyreceived the data packet, then that data packet is retransmitted.Accordingly, the data packet is retained in the buffer 20 untilconfirmation has been received that a packet has been correctlyreceived. It is also possible to use a window method. In this method, apredetermined number of packets are transmitted and an acknowledgementis only required after the predetermined number of packets have beentransmitted.

The mobile station MS1 now enters a zone 24 which is adjacent to thehandoff area S. As can be seen from FIG. 3, the difference between thepower level of the signal received from the first base transceiverstation BTS1 and the second base transceiver station BTS2 at the mobilestation MS1 is becoming smaller. As soon as the mobile station entersthis zone 24, the mobile station sends a signal to the first basetransceiver station BTS1 which causes that first base transceiverstation BTS1 to send a signal to the base station controller BSC2. Thissignal is a message sent through the first base station BTS1 to the basestation controller BSC2 which informs the base station controller thatthe mobile is now registered in both cells. This signal thereforeprovides a cell update for both the cell handled by BTS1 and the cellhandled by BTS2. That signal causes the base station controller BSC2 totransmit the data packets both to the first base transceiver stationBTS1 and to the second base transceiver station ETS2. This is becausethe base station controller BSC2 considers that the mobile station MS1is reachable through both the first and second base stations BTS1 andBTS2. However, only the first base transceiver station BTS1 willtransmit the data packets to the mobile station MS1 whilst the mobilestation MS1 is in zone 24. It should be noted that the buffer 20provided in each base transceiver station BTS1-3 has a finite size sothat packets older than a given threshold will be discarded.Accordingly, whilst the first base transceiver station BTS1 istransmitting the data packets to the mobile station MS1, the second basetransceiver station BTS2 will discard the older data packets at a ratewhich corresponds to the rate at which data packets are received by thesecond base transceiver station STS2.

In one alternative, the first base station BTS1 could advise the secondbase station BTS2 of the proper receipt of a packet directly if the twobase stations are connected or indirectly via the base stationcontroller BSC2. In the latter situation, the base station controllerBSC2 forwards the indication to the second base station BTS2. The secondbase station STS2 will remove the acknowledged packet from its buffer.

As the mobile station MS1 continues to move towards the second basetransceiver station BTS2, the mobile station will enter the handoff zone8. As shown in FIG. 3, the power level of the signal received at themobile station MS1 from the first and second base transceiver stationswill be of a similar level. Handoff will occur at some point as themobile station MS1 travels through this zone. The base stationcontroller BSC2 continues to transmit the data packets to both the firstbase transceiver station BTS1 and the second base transceiver stationBTS2. However, the first base transceiver station BTS1 will stoptransmitting data packets to the mobile station MS1 and the second basetransceiver station BTS2 will start transmitting packets to the mobilestation MS1.

It should be noted that in terms of the data packet communication, hardhandoff will be operated. In other words, the connection between themobile station MS1 and the first base transceiver station BTS1 will beterminated before the connection is made between the mobile station MS1and the second base transceiver station BTS2. When the connectionbetween the first base transceiver station BTS1 and the mobile stationMS1 is terminated, the mobile station will know the identity of the lastcorrectly received packet from the first base transceiver station. Afterthe connection has been established with the second base transceiverstation BTS2, the mobile station MS1 will send the identity of the lastcorrectly received packet to the second base transceiver station BTS2.The identity sent to the second base transceiver station BTS2 by themobile station MS1 may be encoded so as to prevent corruption and mayinclude, for example check digit information or the like.

The second base transceiver station BTS2 will then discard all thepackets in its buffer 20 up to and including the last identified datapacket. The second base transceiver station BTS2 will then starttransmitting data packets from the buffer 20. In particular, the secondbase transceiver station BTS2 will send the data packet immediatelysucceeding the last packet correctly received by the mobile station MS1from the first base transceiver station BTS1. Any packet which requiresretransmission may not be discarded and instead may be retransmitted.

It should be noted that there is no need to advise the base stationcontroller that handoff has occurred between the first and second basetransceiver stations BTS1 and BTS2. This is because the base stationcontroller BSC2 continues to send data packets to both the first and thesecond base transceiver stations BTS1 and BTS2.

Zone 26 is similar to zone 24 in that the data packets will be sent toboth the first and the second base transceiver stations BTS1 and BTS2and in that only one base transceiver station, in this case the secondbase transceiver station BTS2, transmits to the mobile station MS1. Ascan be seen from FIG. 3, the level of the signal received at the mobilestation MS1 from the second base transceiver station BTS2 is increasingrelative to the received level of the signal from the first basetransceiver station BTS1.

When the mobile station reaches the edge of zone 26 and moves intoregion 28, the level of the signal received from the second basetransceiver station BTS2 will be very much higher than that receivedfrom the first base transceiver station BTS1. This can be seen from FIG.3. Accordingly, at this point the mobile station MS1 will advise thesecond base transceiver station BTS2 that the edge of zone 26 has beenreached. This information is passed on to the base station controllerBSC2 which stops sending the data packets to the first base transceiverstation BTS1 and instead sends the data packets only to the second basetransceiver station BTS2.

The zone defined by regions 24, 8 and 26 constitutes a multicast zone inwhich the data packets are transmitted to both the first and the secondbase transceiver stations BTS1 and BTS2.

There are a number of different ways in which the zones 24, 8 and 26 canbe defined. For example, these zones may be predefined. In thisembodiment, the location of the mobile station with respect to the firstand second base transceiver stations BTS1 and BTS2 would be determined.For example, region 24 would occur when the mobile station is between xand y meters from the first base transceiver station and a and b metersfrom the second base transceiver station BTS2. In other words, theregions have fixed geographic locations with respect to the first andsecond base transceiver stations BTS1 and STS2.

However, a more preferred method of defining these regions is to rely onreceived level of the signals from the respective base transceiverstations. Thus, the beginning of region 24 occurs when the receivedpower level of the signal from the first base transceiver station BTS1is less than the value P1. Likewise, the end of region 26 is defined asoccurring when the power level of the signal received from the secondbase transceiver station exceeds the value P1. Alternatively, thebeginning of region 24 can be defined as the point when the signalreceived from the second base transceiver station exceeds the thresholdlevel P2. Likewise, the end of region 26 can be defined as occurringwhen the signal received from the first base transceiver station fallsbelow level P2.

The beginning of the handoff region 8 can be defined as occurring whenthe level of the signal received from the first base transceiver stationBTS1 falls below the power level P3, which is between levels P1 and P2.Likewise, the end of the handoff region 8 can be defined as when thesignal received from the first base transceiver station BTS1 has fallenbelow the value P4 where P4 is between values P3 and P2.

In embodiments of the present invention, at is not necessary to use boththe values of P1 and P2. Whilst these values have been shown asoccurring at the same time for the received power levels of the signalsfrom the different base transceiver stations, in is practice this maynot occur and accordingly, it is more preferably to use the value P1which is indicative that the signal from the closer base transceiverstation has exceeded a given value. In summary, the decision as to thelocation of the beginning and end of each zone may be defined only bythe levels of the signals received from the first base station or thesecond station.

Alternatively the beginning and/or end of at least one zone may bedefined only by the power level of the signals received from the firstbase transceiver station BTS1 whilst the beginning and/or end of atleast one zone may be defined only by the power level of the signalsreceived from the second base transceiver station. Additionally oralternatively both the received power level of signals from both thefirst and second base transceiver stations may be used to define thebeginning and/or end of a zone.

In an alternative embodiment of the present invention, instead oflooking at the absolute power levels of the signal received at themobile station MS1 from the first and/or second base transceiver stationSTS1 and BTS2, the ratio between those signals can be considered. Forexample, the beginning of the region 24 may be defined as occurring whenPRBTS1/PRBTS2 ≦R₁, where PRBTS1 is the power level of the signalreceived from the first base transceiver station at the mobile stationMS1 and PRBTS2 is the power level of the signal from the second basetransceiver station received at the mobile station MS1. R₁ is athreshold value. Likewise, the end of zone 26 may be defined byPRBTS2/PRBTS1 >R₂ where R₂ is a different threshold value. The beginningand end of the handoff region 8 can be defined in a similar manner.

It is of course possible to use other parameters in order to determinethe location of the various regions. For example, instead of powerlevel, signal quality or quantity may be used or the values may be basedon the amount of traffic. Alternatively, the location of the regions canbe determined based on the quality of service required by a mobilestation. This may depend on the application required by the mobilestation. For example speech has a lower requirement than data. Theregions may be therefore smaller for data than speech.

For those embodiments of the present invention which rely onmeasurements of conditions within the cellular network in order todefine the location of the regions, it is preferred that decisions bemade on the basis of average values rather than instantaneous values.For example, in the case where the power level of the signals receivedfrom the first and/or second base transceiver stations are used in orderto determine the location of the region, the level of the signals usedis based on an average. This is so as to avoid anomalous increases ordecreases in the signal from providing false results. The level of thesignal may be averaged over a predetermined number of frames.

It should be noted that the same protocol relating to theacknowledgement and retransmission of incorrectly received data packetsalso takes place in zones 24, 8 and 26.

It should be noted that the transmission of the data packets may takeplace within allocated slots in frames. However, it is also possiblethat the timing of the sending of the data packets be independent of thetime slot and frame structure.

The power level measurements may be based on the power level of the datapackets received from the base transceiver station or alternatively maybe based on other channels which may be in operation at the same timesuch as a control channel or the like.

The size of the buffer can be determined based on trial and errormeasurements. The buffer 20 may be a FIFO.

It should be noted that in embodiments of the present invention, thereis no reason why the regions 24 and 26 should be of the same size. Oneor other of those regions may be larger than the other. However, inpractice it is likely that both of these regions will be of a similarsize.

The base switching center BSC2 may be arranged to contain a routingtable which stores the base transceiver stations to which the datapackets are to be broadcast.

It should be appreciated that whilst the present invention has beendescribed in the context of a GSM system, embodiments of the presentinvention are equally applicable to other access systems including othertypes of time division multiple access systems, frequency divisionmultiple access systems, base division multiple access systems, spreadspectrum multiple access systems such as code division multiple accessand hybrids thereof. In one implementation of embodiments of the presentinvention in code division multiple access systems, whilst soft handoffmay be used, the packets which are to be transmitted may or may not betransmitted to the mobile station by more than one base transceiverstation at a time. Soft handoff is where the same signal is received bya mobile station from more than one base transceiver station.

The embodiment described hereinbefore shows the simultaneoustransmission of data packets to two base transceiver stations at thesame time. It is of course possible that more than two base transceiverstations could receive the same data packets. It is also possible thatthe first and second base transceiver stations could receive the samedata packets with the following data packets being received by thesecond and a third base transceiver station.

Where the adjacent base transceiver stations are not controlled by acommon base station controller, the same data packets would then besupplied to two different base station controllers which then, whenappropriate, would supply the two adjacent base transceiver stationswith the same data packets.

In one modification to the embodiment described hereinbefore, the bufferin the base transceiver stations could be omitted and instead includedin the base station controller. In this modification, theacknowledgement which the base transceiver station receives from themobile station would then be transmitted to the base station controller.If a packet needed to be retransmitted, the packet would be sent againto the base transceiver station. Thus, when handoff occurs, the basestation controller would have the identity of the last acknowledgedpacket received by the mobile station. The base station controller wouldthen be able to send to the new base transceiver station, the next datapacket to be transmitted.

Reference will now be made to FIGS. 6 and 7 which show an alternativeimplementation of the present invention in a general packet radioservice (GPRS) network. Referring first to FIG. 6, a mobile station 100is arranged to communicate with more than one base station 102 and 104,for example when in soft handoff. The two base stations 102 and 104 areconnected to the same base station controller 105 which is connected toa serving GPRS support node (SGSN) 106. The SGSN 106 is connected to agateway GPRS support node 108 (GGSN). The SGSN 106 stores information.Identifying the or each cell in which the mobile station 100 iscurrently located. When the mobile station 100 is able to communicatewith more than one cell and has registered in more than one cell, themobile station is considered to be in more than one cell by the SGSN.The description hereinafter will consider the simplified case where themobile station communicates with only two base stations, which occurswhen the mobile station is located in region 8 of FIG. 1. The mobilestation may communicate with more than two base stations at the sametime. When a data packet is to be transmitted to the mobile station 100,the SGSN 106 will forward the packet, received from the GGSN 108, to thebase station controller 105 which will in turn forward the packet toboth of the base stations 102 and 104 which serve the respective cellsin which the mobile station 100 is located. In this embodiment, both ofthe base stations 102 and 104 are controlled by the same base stationcontroller 105. Accordingly the SGSN 106 only sends the packet once tothe base station controller 105 along with an indication that the packethas to be forwarded to both of the base stations 102 and 104. One orboth base stations 102 and 104 will transmit the same packet to themobile station 100. The packets received by the mobile station 100 maybe acknowledged via the base station 102 or 104 from which the firstreceived packet is transmitted. Alternatively, the acknowledgement canbe sent via either or both of the base stations, regardless of whichbase station transmitted the acknowledged packet. The base station 102and/or 104 transmits the acknowledgement to the base station controllerand/or SGSN 106. It should be noted that the acknowledgement confirmsthat the packet has been correctly received.

When the base station controller or the SGSN 106 receives theacknowledgement from the respective base station 102 or 104, it willknow that the data packet has been correctly received. The data packetsmay be buffered in the base stations 102 and 104, the base stationcontroller 105 or SGSN 106. In either case, the receipt of anacknowledgement will cause the packet in question to be removed from thebuffering of the base station the base station controller or the SGSN108. Typically, in a time division multiple access system (TDMA), amobile station communicates with one base station at a time whilst in acode division multiple access system (CDMA), a mobile station cancommunicate with more than one base station at the same time. In bothaccess systems, if uplink packets are correctly received by only onebase station, the base station controller or SGSN connected thereto candeduce that the best path to the mobile station is likely to be via thatone base station. If or when the mobile station receives the same packetfor the second time, the mobile station will identify that the packethas already been received and delete that second packet and will notacknowledge that second packet.

In the GPRS network, each data packet is provide with a uniqueidentifier. The mobile station 100 can use this identifier to determinewhen duplicate packets are received. Each packet may include informationidentifying the base station from which the data packet has beenreceived.

It is alternatively possible that a mobile station can pass theacknowledgement to any of the base stations with which it is registeredor with which it is communicating and not just the base station fromwhich it first received a given data packet. For example, in a GPRSnetwork, a mobile station might have acknowledged a packet from firstbase station and then move to the cell associated with a second basestation. When this occurs, the mobile station may resend theacknowledgement to the second base station so that the second basestation deletes the acknowledged packet from its buffer and thentransmits the next packet to the mobile station.

Reference will now be made to FIG. 7 which also shows a possiblearrangement in the GPRS network. In this arrangement, the mobile station100′ is also registered or in communication with more than one basestation 102′ and 104′. In other words each data packet is transmitted byboth of the base stations 102′ and 104′. Unlike the arrangement of FIG.6, each of the two base stations 102′ and 104′ are connected viadifferent base station controllers 105 a and 105 b to different SGSNs106 a and 106 b. Each of the SGSNs 106 a and 106 b is connected to aGGSN 108′. There are three ways in which this arrangement could operatein practice.

In the first method of operation, the acknowledgement layer is extendedback to the GGSN 108′. In other words when the first packet to bereceived by the mobile station 100′, an acknowledgement is sent by themobile station 100′ to the base station which has sent the firstreceived packet. That base station passes the acknowledgement on to theSGSN to which that base station is connected via the associated basestation controller. That SGSN then passes the acknowledgement to theGGSN 108′. The GGSN 108′ will then know that the packet has beencorrectly received. If a data packet is to be retransmitted, this willbe done by the GGSN 108′. Thus in the arrangement of FIG. 6 the controlof the transmission and retransmission of the data packets is controlledby the SGSN to which both the base stations are connected. In contrast,the first method of operation of the arrangement of FIG. 7 the GGSN 108′controls the transmission and retransmission of data packets.

In the second method of operation, the acknowledgement mechanism isextended between the GGSN 108′ and the mobile station 100′ butcoordination between the SGSNs 106 a and 106 b and the GGSNs 108′ isprovided. Retransmission may be controlled by the SGSNs. The GGSN 108′includes in each data packet to be transmitted a first identificationnumber. Each SGSN 106 a and 106 b then adds a second identificationnumber (logical link control LLC number) to each data packet. When themobile station 100 sends its acknowledgement it include both the firstand second identification numbers. This can then be dealt with in twoways. In the first way, the SGSN receiving the acknowledgment from themobile station (via a base station and base station controller) sends anacknowledgement using the first identification number to the GGSN 108′.The GGSN 108′ then advises the other SGSN that the data packetidentified by the first identification number has been correctlyreceived.

Alternatively, the SGSN receiving the acknowledgement can advise theother SGSN directly that the data packet has been received. Thisrequires a connection between the SGSNs. It also requires that the SGSNsare able to identify other SGSNs to which a mobile station is coupledfrom, for example, the second identification number. The acknowledgementcan thus be at the LLC level (logical link control) which means that theacknowledgement is from the mobile station to the GSN. Theacknowledgement may alternatively be at the RLC level (radio linkcontrol) which is from the mobile station to the base station and/or tothe base station controller.

In one embodiment, one SGSN 106 a or 106 b is designated as the anchorSGSN. The anchor SGSN can also be referred to as a dominant node. Theother SGSN is used only to route data. The anchor SGSN will have all thesubscriber data and perform various GPRS functions such as SNDCP(subnetwork dependent convergence protocol) compression and LLC (logicallink control) ciphering. The data packets received by the anchor SGSNare thus suitably processed and forwarded to the base station connectedthereto via the appropriate base station controller. The data packet isalso forwarded to the other SGSN by the anchor SGSN which in turnforwards the packet to the other base station which is in communicationwith the mobile station, again via the appropriate base stationcontroller. The other SGSN thus stores information so that it canidentify the base station controller and the base station to which agiven packet should be forwarded. The SGSN also stores information as tothe anchor SGSN for each mobile station in a cell associated with thatSGSN. All acknowledgements of the data are forwarded to the anchor SGSNeither via the base station and base station controller connectedthereto or from the other SGSN.

It should be appreciated that this method requires a mobile station todesignate one of the base stations to which it is connected as the mainbase station. The main base station is the base station connected to theanchor SGSN. The mobile station will only deregister from its anchorbase station if a connection is poor. If the mobile station changes itsanchor base station and/or a secondary base station to which it isconnected, it is necessary to ensure that the system carries out thenecessary updates so that the anchor and other base stations arecorrectly identified.

In a modification to the embodiments described in relation to FIG. 7, anetwork (eg ATM) is arranged between all of the SGSNs and all the basestations. With this modification, each base station just needs to knowto which SGSN the mobile station is connected. Each base station whichis connected to a mobile station will each transmit the same datapackets to mobile station. Each base station which is connected to thesame mobile station will forward acknowledgements from the mobilestation to the associated SGSN. This method operates in a similar mannerto that shown in FIG. 6.

In the arrangements described in relation to FIG. 7, data packets willbe buffered in the base stations 102′ and 104′ and/or the base stationcontroller 105 a and 105 b and/or the SGSNs 106 a and 106 b. The receiptof an acknowledgement of a particular data packet will cause the datapacket to be removed from buffering in the base stations 102′ and 104′and/or the SGSNs 106 a and 106 b.

In the embodiments of FIGS. 6 and 7, data packets will be retransmittedif an acknowledgement is not received within a predetermined time. Inboth of the arrangements shown in FIGS. 6 and 7, a mobile station isarranged to register with a base station if that mobile station is inits cell. As will be apparent from the previous description, a mobilestation can be in more than one cell at the same time. When a mobilestation leaves a cell, the mobile station will deregister from thatcell. Depending on the implementation of the embodiments of the presentinvention, the registration of the mobile station with a given basestation can be passed to one or more SGSNs and/or the GGSN and/or thebase station controllers. If the two base stations are connected to twodifferent SGSNs, the mobile station may be registered with two differentSGSNs. This information will be stored by or at least available to theGSN. If the cells belong to two different routing areas, the registeringand deregistering signals will update both of the associated SGSNs, ifthe two routing areas are associated with different SGSNs. In thisarrangement, the SGSNs provide area controllers. However, in alternativeembodiments of the present invention, the controller can be provided inany suitable location. It should be appreciated that a routing areagenerally comprises a plurality of cells and constitutes the area ofwhich paging takes place in GPRS. A routing area is similar to thelocation area of the GSM standard.

It should be appreciated that in implementation of the present inventionboth of the arrangements of FIGS. 6 and 7 may be included in the samenetwork. For convenience, in both of the arrangements shown in FIGS. 6and 7, the mobile station is shown as communicating with two basestations at the same time. Each mobile station can of course communicatewith more than two base stations at the same time in certaincircumstances.

In the embodiments described in relation to FIGS. 6 and 7, it is assumedthat the mobile station might be registered in two cells or in a softhandoff situation. However, at other times, the mobile station will onlybe in a single cell. When the mobile station is on the border with asecond cell, the mobile station will send a request to see if the mobilestation can now additionally register with the base station associatedwith that other cell. The mobile station will then be advised as towhether or not a connection can be made to the base station of thesecond cell before the mobile station attempts any such connection.

If such a connection is permitted, any necessary connections andexchange of information between SGSNs and the GSN as well as betweenSGSNs themselves can be set up before the mobile station makes anyconnection with the base station associated with the second cell.Packets of data could thus be forwarded to the base station of thesecond cell or the associated SGSN before the connection between themobile station and the base station of the second cell has beenestablished.

In one possible implementation, the mobile station sends along withupdate information to the registers, a list of data packets which havebeen received so that the correct subsequent data packets are sent bythe second base station. This feature is particularly advantageous in ahard hand off situation where the mobile station is in communicationwith only one base station at the same time.

It should be appreciated that the arrangement shown in relation to FIGS.6 and 7 can be modified to work in a hard handoff situation as well asor instead of in the soft handoff situation discussed hereinbefore,similar to that described in relation to FIGS. 1 to 5. In particular, ina CDMA system the base stations may be arranged so that only one basestation at a time transmits data packets. However, more than one basestation could receive the packets at the same time. The term basestation controller is sometimes associated with the GSM standard. Inother systems such as the CDMA system, a radio network controllerprovides a similar function to that described in relation to the basestation controller hereinbefore. The term base station controller shouldin the context of this document be interpreted to include base stationcontrollers as well as radio network controllers.

Aspects of the arrangements described in relation to the embodimentsdescribed in relation to FIG. 1 to 5 may be incorporated in thearrangements described in relation to FIGS. 6 and 7 and vice versa.

In the context of this document, the term connection can be a wiredconnection or a wireless connection. Direct connections may be providedbetween base stations in some embodiments of the present invention.

Embodiments of the present invention are applicable to any hierarchicalwireless or wired communication system. The packet data may be sent totwo station base controllers instead of to two base stations.

The embodiments described hereinbefore have been in the context of awireless network. However embodiments of the invention may also beapplicable to wired networks.

1. A method comprising: transmitting data packets from a common node tofirst and third stations; receiving at said common node anacknowledgement forwarded to the common node by one of first and secondnodes; and advising the other of the first and second nodes that saidacknowledgment has been received, wherein said first station is incommunication with said first node, said third station is incommunication with said second node, said first and second nodes are incommunication with said common node, and said third station isconfigured to send data packets to a second station commencing with apacket identified as being required after a last correctly receivedpacket received at the second station from the first station.
 2. Amethod as claimed in claim 1, further comprising: receiving at the thirdstation from the second station acknowledgement of correct receipt ofthe packet identified as being required by the second station.
 3. Amethod as claimed in claim 2, further comprising: transferring saidacknowledgement to the common node.
 4. A method as claimed in claim 1,further comprising: receiving at the third station an identification ofthe last packet correctly received by said second station from the firststation, and transmitting from the third station to the first station apacket subsequent to the last correctly received packet.
 5. A method asclaimed claim 1, further comprising: receiving at the third stationcopies of packets of data received at the first station for transmissionto the second station.
 6. A method as claimed in claim 5, furthercomprising: receiving said copies of data packets when the secondstation has at least one predefined parameter with respect to said firstand third stations.
 7. A method as claimed in claim 6, wherein saidpredefined parameter is defined by the geographic position of the secondstation with respect to the first and third stations.
 8. A method asclaimed in claim 6, wherein said at least one predefined parameter isdefined by at least one parameter of signals received from at least oneof said first and third stations satisfying a predetermined criteria. 9.A method as claimed in claim 8, wherein said signal parameter is thepower level of a signal received at the second station from at least oneof said first and third stations.
 10. A method as claimed in claim 8,wherein said signal parameter is the ratio of power level of signalsreceived at the second station from the first and third stations.
 11. Amethod as claimed in claim 8, wherein said signal parameter is thequality of the signal received at the second station from at least oneof said first and third stations.
 12. A method as claimed in claim 8,wherein said signal parameter is the quantity of traffic associated withat least one of said first and third stations.
 13. A method as claimedin claim 8, wherein said parameter is averaged over time before it isdetermined when said criteria is satisfied.
 14. A method as claimed inclaim 6, wherein said predefined parameter is the quality of signalsassociated with at least one of said first and third stations.
 15. Amethod as claimed in claim 6, wherein said predefined parameter definesa handoff zone in which said first station hands off to said thirdstation in that second station stops receiving data packets from saidfirst station and starts receiving data packets from said third station.16. A method as claimed in claim 15, wherein said handoff zone issurrounded on either side by a zone in which the second station receivesdata packets from a respective one of said first and third stations andsaid first and third stations are provided with the same data packets.17. A method as claimed in claim 1, further comprising: receiving at thethird station from said second station a signal advising the thirdstation of which packet or packets were received at the second stationfrom the first station, and transmitting from said third station thedata packet identified as being required after the last packet to saidsecond station.
 18. A method as claimed in claim 1, wherein when saidsecond station does not correctly receive a data packet, said secondstation requests retransmission of said data packet.
 19. A method asclaimed in claim 1, further comprising: storing data packets at thethird station when received thereat.
 20. A method as claimed in claim19, further comprising: discarding at the third station, the datapackets preceding a next data packet to be transmitted.
 21. A method asclaimed in claim 1, wherein the common node associates a unique numberfor each packet and the same data packets, each of which is associatedwith the unique number are transmitted from said common node to both ofthe first and third stations.
 22. A method as claimed in claim 1,wherein the common node controls the removal of said identified datapacket from a data storage.
 23. A method as claimed in claim 1, whereinthe first station is connected to a first node and the third station isconnected to a second node and one of the first and second nodes isarranged to be the common node and acknowledgements are forwarded to thecommon node.
 24. A method as claimed in claim 1, wherein the first andsecond nodes and/or the first and third stations are connected together.25. A method as claimed in claim 1, wherein the first and third stationsare connected to the common node via a network.
 26. A method as claimedin claim 1, wherein a data storage is provided in said common node. 27.A method as claimed in claim 1, wherein said transmitting is in acommunication network, and wherein said network is a wireless network.28. A method as claimed in claim 1, wherein said first and thirdstations are base stations.
 29. A method as claimed in claim 1, whereinthe first and third stations are base station controllers.
 30. A methodas claimed in claim 1, wherein said first and third stations are part ofa general packet radio system and said common node is a serving generalpacket radio service support node.
 31. A method as claimed in claim 30,wherein packets of data are forwarded to at least one serving generalpacket radio service support node prior to any connection beingestablished with said second station.
 32. A method as claimed in claim1, wherein said first and second nodes are serving general packet radioservice support nodes of a general packet radio service network and saidcommon node is a gateway general packet radio service support node. 33.A method as claimed in claim 1, wherein said common node is a basestation controller.
 34. A method as claimed in claim 1, wherein thecommon node is not advised of the occurrence of handoff.
 35. A method asclaimed in claim 1, wherein said second station is a mobile station. 36.A method as claimed in claim 1, wherein said second station is able toregister with one or more of a plurality of cells or areas of acommunication network at the same time.
 37. A method as claimed in claim1, wherein packets of data are forwarded to at least one of said firstand third stations before a connection is made with said second station.38. A system, comprising: first, second and third stations, wherein saidfirst station is configured to send a first number of data packets tothe second station, the second station is configured to identify whichof the first number of packets it receives from the first station, andthe third station is configured to send a second number of data packetsto the second station commencing with a data packet identified as beingrequired after a last correctly received packet from the first station;a first node in communication with said first station; a second node incommunication with said third station; and a common node incommunication with said first and second nodes, wherein anacknowledgement is forwarded to the common node by one of said first andsecond nodes, and the common node is configured to advise the other ofthe first and second nodes that said acknowledgement has been received.39. An apparatus, comprising: a transmitter configured to transmit datapackets from the apparatus to first and third stations; a receiverconfigured to receive at the apparatus an acknowledgement forwarded fromone of first and second nodes; and a controller configured to advise theother of the first and second nodes that said acknowledgment has beenreceived, wherein said first node is in communication with said firststation, said second node is in communication with said third station,said first and second nodes are in communication with said apparatus,and said third station is configured to send data packets to a secondstation commencing with a packet identified as being required after alast correctly received packet received at the second station from thefirst station.
 40. An apparatus as claimed in claim 39, furthercomprising: a memory configured to store data packets prior totransmission from said third station to said second station.
 41. Anapparatus as claimed in claim 40, wherein said memory comprises abuffer.
 42. An apparatus as claimed in claim 39, wherein said thirdstation is configured to receive copies of data packets received at thefirst station for transmission to said second station.
 43. An apparatusas claimed in claim 39, wherein the controller is configured toassociate a unique number for each packet and the same data packets,each of which is associated with the unique number are transmitted fromsaid apparatus to both of the first and third stations.
 44. An apparatusas claimed in claim 39, wherein the controller is configured to controlthe removal of said identified data packet from a data storage.
 45. Anapparatus as claimed in claim 39, further comprising a data storage atsaid apparatus.
 46. An apparatus as claimed in claim 39, wherein saidfirst and third stations are part of a general packet radio system andsaid apparatus is a serving general packet radio service support node.47. An apparatus as claimed in claim 39, wherein said first and secondstations are serving general packet radio service support nodes of ageneral packet radio service network and said apparatus is a gatewaygeneral packet radio service support node.